2.0 Analysis 2.1 Carburettor Heat The atmospheric conditions were conducive to light carburettor icing at cruise or descent power. As the aircraft engine was operating at either take-off or climb power from take-off until the loss of engine power, the formation of carburettor icing was unlikely. Even though carburettor heat was not required by the POH, the pilot applied carburettor heat during the climb, shortly before the engine lost power. A slight reduction in rpm was observed, indicating that carburettor heat was reaching the engine. This application of carburettor heat further reduces the likelihood of carburettor icing being a causal or contributing factor in this occurrence. 2.2 Fuel Tank Design The rags found in the tanks sank when placed in fuel. Since the fuel tank outlets are at the lowest point of the fuel tanks, the rags, when placed in the tank, would have gravitated toward the fuel tank outlet. Given that the filler neck affords a view of its immediate area only, the rags would have been out of the sight of the pilot carrying out a pre-flight check. 2.3 Fuel Starvation Once the engine was started and the aircraft was moving, the rags would have been drawn to the fuel tank outlets by the additional effects of fuel movement in the tanks and fuel flow out of the tanks to the engine. The rags collected at the fuel tank outlet and restricted the flow of fuel to the engine, leading to fuel starvation and loss of engine power. When the aircraft struck the tree after the first power loss, the fuel in the fuel tanks probably moved abruptly, dislodging the rags from the fuel outlets and allowing a temporary resumption of fuel flow and engine power before the rags again migrated to the fuel outlets. 2.4 Aircraft Security Since the aircraft had been flown about four hours before the accident and the rags sink in fuel, it is unlikely that the rags entered the fuel tanks before the aircraft left its base. As the pilot used no rags while operating or refuelling the aircraft, the rags were probably put in the tanks while the aircraft was parked at Little Grand Rapids. Since no screens were fitted on the filler necks of the fuel tanks, rags and other foreign objects could be introduced into the fuel tanks. Once in a tank, any object would likely move out of view and would then be undetectable during pre-flight inspection. 2.5 Airport Security The aircraft was parked on the ramp from 1700 on April 22 to 1400 on April 24. The terminal building was closed from the time the aircraft arrived until one hour before it departed. While the terminal building is closed, no staff are present; therefore, there is no one to observe the movements of persons on the ramp. Since the airport fencing at Little Grand Rapids Airport has an unlocked gate, unauthorized persons may gain unsupervised access to the aircraft parked on the ramp. 2.6 Injuries Significant reductions in injuries can be achieved by using shoulder harnesses. The pilot's injuries during the forced landing might have been avoided by use of the available shoulder harness. 3.0 Conclusions 3.1 Findings The pilot was certified and qualified for the occurrence flight in accordance with existing regulations. The aircraft's records indicate that the aircraft was certified and maintained in accordance with existing regulations. The aircraft's engine lost power because of fuel starvation resulting from rags placed in the aircraft's fuel tanks by a person or persons unknown. The rags were probably placed in the fuel tanks while the aircraft was parked on the ramp at the Little Grand Rapids Airport. The airport fencing allows public access to aircraft parked on the ramp via a gate and via the runway area. There is no supervision of persons on the ramp area after normal working hours or on Saturdays. The aircraft fuel tank design does not incorporate a lock or filler neckscreen, and does not allow easy inspection of the entire fuel tank. The pilot's injuries during the forced landing might have been avoided by use of the available shoulder harness. 3.2 Causes The aircraft's engine lost power because of fuel starvation resulting from rags placed in the aircraft's fuel tanks by a person or persons unknown. Contributing factors were the ease of access by the public to aircraft parked on the ramp, and the lack of security of the aircraft's fuel supply. The Board has no aviation safety recommendations to issue at this time.4.0 Safety Action The Board has no aviation safety recommendations to issue at this time.